Among a large number of UV absorbers, a benzotriazole-type UV absorber which is transparent to visible light is used most widely in terms of amounts and applications. However, conventional UV absorbers involve various problems to be solved. For example, the conventional benzotriazole-type UV absorbers have the following drawbacks.
(1) They are high in vapor pressures because of the low molecular weights. When a conventional UV absorber is blended with a resin and molded, it tends to volatilize, so that the yield is decreased or the contamination of mold or the pollution of working environment occurs. Further, the absorber bleeds out of the surface of molded article or coating film with the lapse of time to impair the appearance of the product. It is washed out by rain or water containing a detergent or the like in using environment, failing to impart UV absorption to products over a long period of time.
(2) The benzotriazole-type compound has inherently a property to react with a wide variety of metal ions(metal ion reactivity). For this reason, it easily forms chelate compounds with metal ions, so that color formation occurs, impairing an inherent transparency to visible light and losing properties as UV absorber.
Accordingly, in order to alleviate the defects indicated in (1) and (2) above, namely, volatility, elution, reactivity with metal ions and the like, a method has been usually employed in which a bulky substituent is introduced in the molecule of UV absorber, especially, in an adjacent carbon atom having a phenolic hydroxyl group.
However, the bulky substituent to be introduced in the method is usually a tert-butyl group, a tert-octyl group or a dimethylbenzyl group. The introduction of such a substituent incapable of UV absorption lowers the molecular absorption coefficient in proportion to increased molecular weight of the UV absorber, with the result that UV absorption per molecule is reduced.
Another method is known in which 2-hydroxyphenylbenzotriazole group is introduced as bulky group (Japanese Unexamined Publication No. 49-61,071, Japanese Examined Patent Publication No. 55-39,180 and the like).
Although the compounds disclosed in these documents exhibit improved properties concerning volatility, elution, etc., but the effects achieved are still unsatisfactory. Further, these documents does not teach the importance of metal ion resistance.
Japanese Unexamined Patent Publication No.60-38,411 discloses an approach in which polymerizable unsaturated group (CH.sub.2 .dbd.C(R.sup.a)COO(R.sup.b)--) (wherein R.sup.a is a hydrogen atom or a methyl group, and R.sup.b is a linear or branched alkylene group having 2 to 10 carbon atoms) is introduced into the molecule of UV absorber to convert it into a copolymer and to improve the properties relating to volatility and elution.
However, the polymerizable UV absorber described in the document has a defect that it does not sufficiently absorb UV and has a high volatility itself, so that it is volatilized when the graft polymerization reaction is conducted, for example, in an extruder or a kneading molding machine.
Accordingly, a UV absorber has not been found yet which can eliminate the defects associated with the conventional UV absorber, namely, which is free from volatility and elution problems during polymerization or molding, which is excellent in alkali resistance, heat resistance and metal ion resistance and which has a high UV absorptivity. Consequently, it is desired to provide UV absorbers having been improved in these properties.